This invention relates to an easily manufactured infrared optical fiber capable of low-loss transmission of infrared light.
The core material employed in optical fibers for infrared transmission requires surface protection since it is soft and easily flawed. A conventional structure for realizing such surface protection is illustrated in FIG. 1, and comprising a cladding 2a of a polymer material which is used to clad a core 1a. This so-called "polymer-clad" fiber is well-known in the art, and is known to exhibit certain short-comings. Specifically, the surface of the core 1a is likely to be flawed as it is inserted into the polymer cladding 2a. If the core is to carry a highpower CO.sub.2 or CO laser beam and the laser beam should happen to leak into the polymer cladding 2a from the core 1a, a dangerous situation could arise in which the polymer cladding may burn due to the generation of heat. A number of arrangements have been proposed in an effort to preclude the aforesaid difficulty. One such arrangement is illustrated in FIG. 2 and makes use of spacers 1b which are disposed at suitable intervals axially of the core 1a between the core 1 a and the polymer cladding 2a so as to prevent them from coming into contact, the spacers having a refractive index which is lower than that of the core 1a. The problem encountered in this case, however, is the great difficulty involved in manufacturing the optical fiber while at the same time maintaining the peculiar arrangement of the constituents without the surface of the core being flawed. It is not an overstatement to say that the manufacture of very long fibers having this structure is virtually impossible.
Another proposal, as shown in FIG. 3, contemplates an arrangement which is similar to that of the clad core structure employed in optical fibers made of glass, in that the core 1a is surrounded by a tubular cladding 2b the refractive index of which is lower than that of the core, the infrared light being made to reflect off the core-cladding interface as it propagates along the fiber, and in that the cladding 2b is surrounded by a polymer tube 3a to form a structure that protects both the core 1a and the cladding 2b. In a fiber of this structure which relies upon a metallic halide, however, it is not possible to avoid the scattering loss of infrared light attributed to abrasion at the core-cladding interface even if the fiber is fabricated by an extrusion process. To be more specific, the clad core structure can be manufactured with facility by a drawing technique if the core is made of sillica glass or a like material, but the drawing process cannot be applied to the production of optical fibers that transmit infrared light because such fibers employ a core material which has a low viscosity at its melting point.
In view of the above limitation, the process which has been adopted to manufacture the optical fiber having the structure illustrated in FIG. 3 includes two separate extrusion steps, one for manufacturing the core 1a and the other for manufacturing the tubular cladding 2b, with the formed core then being inserted into the completed cladding 2b. In practice, however, great difficulties are encountered when inserting the core 1a, having an extremely small diameter measured microns, into the cladding 2b, and when inserting the clad core into the protective polymer tube 3. The result is not only a low level of manufacturing efficiency but also unavoidable scattering loss of optical energy due to abrasion at the core-cladding interface caused by the sliding contact between them when the core 1a is inserted.